1. Field of the Invention
The present invention relates generally to portable waste liquid collection devices, and more particularly to a portable tank apparatus adapted to load and unload thick liquids.
2. Description of the Prior Art
There have been heretofore proposed tank trucks for loading, transporting and discharging thick industrial waste liquids, such as heavy crude oil, ship bilgewater and the like. Apparatus of this general nature are disclosed in U.S. Pat. No. 2,664,911 issued Jan. 5, 1954 to Clarence S. Thompson and George E. Clark, and U.S. Pat. No. 3,315,611 issued Apr. 25, 1967 to David L. Thompson. Applicant is also aware of U.S. Pat. Nos. 1,587,864; 3,267,509; 3,378,025; and 3,621,893. Such tank trucks usually include a power-driven pump operable to alternatively furnish either a vacuum or superatmospheric air pressure to the interior of the tank. The tank is provided with a fill pipe through which liquid is drawn into the tank when the pump provides a vacuum within the tank, and with a drain pipe through which the liquid is exhausted from the tank when the pump supplies superatmospheric air pressure to the interior of the tank. The tank is additionally provided with a vent pipe which selectively vents the interior of the tank to the atmosphere. Valves are utilized to control fluid flow through these three pipes.
With the conventional vacuum tank truck heretofore utilized, the flow of liquid out of the tank is controlled by a manually operated drain valve and a separate manually controlled bleed valve. If the operator does not properly coordinate the rate of opening of these two valves, the valve element of the intake or discharge valve can be damaged by rocks or gravel entrained in the liquid being loaded or discharged. Such damage requires costly repair.